Leaving school, getting a job and becoming a mother may all lead to weight gain, research suggests.
Scientists from the University of Cambridge looked at numerous studies to uncover how people pile on the pounds at different stages of life.
They found going to university or getting a job cuts the time people spend exercising by up to 16.4 minutes a day.
The stress of parenthood may also lead to unhealthy habits, with mothers tending to have a BMI 17% heavier than women without children.
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“Children have a relatively protected environment, with healthy food and exercise encouraged within schools,” study author Dr Eleanor Winpenny said.
“This evidence suggests the pressures of university, employment and childcare drive changes in behaviour which are likely to be bad for long-term health.
“This is a really important time when people are forming healthy or unhealthy habits that will continue through adult life.”
The scientists first looked at 19 studies that analysed how the transition from school to higher education or work influences our healthy habits.
Results - published in the journal Obesity Reviews - suggest graduating school reduces the time spent moderately or vigorously exercising by an average of seven minutes.
Men generally become lazier, knocking 16.4 minutes off their daily work out compared to 6.7 minutes for women.
The largest changes occur among university “freshers”, with moderate-to-vigorous exercise falling by 11.4 minutes a day.
Three studies looked at how body weight changes after leaving school, which was insufficient to provide an average figure.
Yet two papers suggest our diets become less nutritious after graduating school or starting university.
In the second part of their research - also published in Obesity Reviews - the scientists looked at how becoming a parent affects our waistline.
After analysing six studies, they found mothers tend to have a heavier BMI than women without children.
A mother of the “average height” 5ft 4inches (164cm) typically gains 8.8kg over the first five-to-six years, a 3.3 increase in BMI.
This is compared to 7.5kg, or a 2.8 BMI increase, among women of the same height but without children.
Only one study looked at fathers’ weight, finding no difference.
“Little evidence” analysed whether a worse diet and less exercise may be to blame.
Of the studies that did look at activity levels, “most” showed a greater reduction among parents.
There was “limited evidence” for diet, which did not seem to differ between those with and without children.
“BMI increases for women over young adulthood, particularly among those becoming a mother,” study author Dr Kirsten Corder said.
“New parents could also be particularly willing to change their behaviour as it may also positively influence their children, rather than solely improve their own health.
“Interventions aimed at increasing parents’ activity levels and improving diet could have benefits all round.
“We need to take a look at the messages given to new parents by health practitioners as previous studies have suggested widespread confusion among new mothers about acceptable pregnancy-related weight gain.”